1. Field of the Invention
The present invention relates to a transflective liquid crystal display device in which each pixel has a light-reflective type reflection area and light-transmissive type transmission area, and to a reflective liquid crystal display device in which each pixel only has a reflection area. Moreover, the present invention relates to terminal devices that are provided with the transflective liquid crystal display device and the reflective liquid crystal display device.
2. Description of the Related Art
As a liquid crystal display device having both a function of a transmissive liquid crystal display device and a function of a reflective liquid crystal display device, there is known a transflective liquid crystal display device (for example, Japanese Unexamined Patent Publication 2003-344837 (Patent Document 1)). This kind of semi-transmissive liquid crystal display device has a transmission area and a reflection area within each pixel. The transmission area transmits light from a backlight source, and uses the backlight source as a light source for display. The reflection area has a reflection plate, and uses light from the outside reflected by the reflection plate as a light source for display.
With the transflective liquid crystal display device, it is possible under bright surroundings to display an image similar to a print by turning on/off the light that reaches the reflection plate from the surroundings. At the same time, it is possible to reduce the power consumption by turning off the backlight source and display an image in the reflection area. Further, it is also possible to display an image in the dark surroundings by turning on the backlight source and displaying an image in the transmission area.
As display modes of liquid crystal display devices, there are lateral electric field modes such as IPS (In Plane Switching) mode and FFS (Field Fringe Switching) mode. A lateral electric field mode liquid crystal display device has a pixel electrode and a common electrode formed on a same substrate, and applies a lateral electric field to a liquid crystal layer. The lateral electric field mode liquid crystal display device can achieve a wider viewing angle compared to a TN (Twisted Nematic) mode liquid crystal display device, by displaying an image through rotating liquid crystal molecules in a direction in parallel to the substrate.
There is disclosed an example of a transflective lateral electric field mode liquid crystal display device which performs normally-black drive by setting retardation of a liquid crystal layer of a reflection area as ¼ wavelength, and setting retardation of a liquid crystal layer in a transmission area as ½ wavelength, and providing a ½ wavelength phase difference layer between a polarizing plate of the reflection area and the liquid crystal layer (For example, Japanese Unexamined Patent Publication 2006-171376 (Patent Document 2)). Further, there is also a transflective liquid crystal display device which performs display by inverting the drive between the reflection area and the transmission area without providing the ½ wavelength phase difference layer (for example, Japanese Unexamined Patent Publication 2007-41572 (Patent Document 3)).
In Patent Document 3, the reflection area and the transmission area of each pixel are provided, respectively, with a common electrode and a switching device for connecting a pixel electrode and a data line to which a data signal is supplied. The reflection area and the transmission area of each pixel are driven with substantially inverted on-off inversion signals, and the liquid crystal molecules are controlled to drive in different directions for the transmission area and the reflection area. The reflection area is normally-white, and the transmission area is normally-black. Thus, it is possible to make displays of both areas to be in bright states by not applying a voltage to the liquid crystal layer in the reflection area and applying a voltage to the liquid crystal layer in the transmission area.
FIG. 53 shows a sectional view of a unit pixel of a transflective liquid crystal display device, which drives the liquid crystal of the reflection area in a lateral electric field mode and the liquid crystal of the transmission area in the lateral electric field mode.
A reflection plate 4 is formed on a lower substrate 3 side of a reflection area 1, and an insulating film 5 is deposited on the reflection plate 4. A reflection common electrode 6 for forming an electric field in the reflection area 1, and a reflection pixel electrode 7 are formed on the insulating film 5. A liquid crystal layer 9 is provided between the lower substrate 3 and a counter substrate 8, and liquid crystal molecules are aligned homogeneously in a direction in parallel to a transmission axis of a polarizing plate 10 on the counter substrate 8 side. Further, retardation of the liquid crystal layer 9 in the reflection area 1 is set as ¼ wavelength, and retardation of the liquid crystal layer 9 in the transmission area 2 is set as ½ wavelength. Furthermore, a transmission pixel electrode 11 and a transmission common electrode 12 are formed on the insulating film 5 in the transmission area 2. Reference numeral 13 is a glass substrate included in the counter substrate 8, and 14 is a glass substrate included in the lower substrate 3.
FIG. 54 shows changes in the polarization state of the reflection area. When there is no electric field generated in the liquid crystal layer 9 of the reflection area 1, display on the reflection area 1 turns out as bright state. Light passing through the polarizing plate 10 becomes linearly polarized light of vertical direction (90 degrees), and the optical axis thereof is in parallel to the alignment direction of the liquid crystal, i.e., in parallel to the major axes of the liquid crystal molecules. Thus, the linearly polarized light passing through the polarizing plate 10 passes through the liquid crystal layer 9 and reaches the reflection plate 4 while keeping the polarized state. In a case of the linearly polarized light, the polarized state does not change even if it is reflected by the reflection plate 4. Thus, the reflected light is also in parallel to the major axes of the liquid crystal molecules. Therefore, even if the reflected light passes through the liquid crystal layer 9 again, it reaches the polarizing plate 10 while keeping the polarized state. Since the optical axis of the reflected light is in parallel to the transmission axis of the polarizing plate 10, the reflected light passes through the polarizing plate 10, thereby providing bright state. In the meantime, where there is an electric field generated in the liquid crystal layer 9 of the reflection area 1, display on the reflection area 1 turns out as dark state. The liquid crystal molecules are rotated within the substrate plane by the electric field, and the angle between the major axes of the liquid crystal molecules and the transmission axis of the polarizing plate 10 becomes 45 degrees. Therefore, the angle between the optical axis of the linearly polarized light passed through the polarizing plate 10 and the major axes of the liquid crystal molecules becomes 45 degrees. Since the retardation of the liquid crystal layer 9 is ¼ wavelength, the linearly polarized light is changed to clockwise circularly polarized light. This clockwise circularly polarized light is changed to counterclockwise circularly polarized light, when reflected by the reflection plate 4. When the counterclockwise circularly polarized light passes the liquid crystal layer 9 again, it is changed to linearly polarized light of lateral direction (0 degree). Since the optical axis of the reflected light is vertical to the transmission axis of the polarizing plate 10, the reflected light cannot pass through the polarizing plate 10, thereby providing dark state.
FIG. 55 shows a result of a simulation conducted regarding alignment of the liquid crystal molecules and the reflectance at the time of dark state, when a voltage is applied to the reflection common electrode 6 and the reflection pixel electrode 7. The lateral electric field that is in parallel to the substrate plane is not applied to the liquid crystal molecules on the reflection common electrode 6 or the reflection pixel electrode 7, so that rotation of the liquid crystal molecules within the substrate plane is insufficient. Thus, even when the light passed through the polarizing plate 10 passes the liquid crystal layer 9, it does not change to a circularly polarized state. That is, the light that passes through the liquid crystal layer 9 on the electrodes 6, 7, makes incident on the reflection plate 4, reflected by the reflection plate 4, and passes the liquid crystal layer 9 on the electrodes 6, 7 comes to be in a polarized state as if it is under no supplied voltage, even though a voltage is being supplied. Thus, the reflected light transmits the upper-side polarizing plate as in the case of bright state. When the electrodes 6, 7 are formed with a transparent electric conductor such as ITO (Indium Tin Oxide), light leaks from the areas on the electrodes 6, 7 even if a voltage is applied to the electrodes 6, 7 to provide dark state. Therefore, the visibility of the reflection display becomes deteriorated.
Further, when the reflection pixel electrode and the reflection common electrode on an uneven reflection plate are formed with the transparent electric conductor, those electrodes cannot be formed with high precision. Photolithography is used for patterning the reflection pixel electrode and the reflection common electrode. The reflection common electrode and the reflection pixel electrode are formed on the reflection plate via an insulating film.
As shown in FIG. 56, the surface of the reflection plate 4 is formed to have uneven shapes by an uneven film 15 so that the reflection plate 4 diffusively reflects the incident light. Thus, when a transparent electric conductor 16 is used as a material for the reflection common electrode 6 and the reflection pixel electrode 7, when resist 17 is exposed through photolithography by using a mask 18 and exposure light 19, the exposure light 19 transmitting through the transparent electric conductor 16 is diffusively reflected by the reflection plate 4. As a result, the resist 17 at an area that is not intended to be exposed becomes exposed as well. Therefore, the shapes of the reflection common electrode 6 and the reflection pixel electrode 7 which are patterned after being developed and etched become unstable. Thus, especially reduction in the electrode widths causes snapping of the wirings of the electrodes.